Historically, lead or lead alloy anodes have been widely employed in processes for the electrowinning of metals, such as copper, from sulphate electrolytes. These lead anodes nevertheless have important limitations such as undesirable power consumption and anode erosion. This anode erosion can lead to sludge production and resulting contamination of one or both of the electrolyte and the electrowon product.
During the time that these lead electrodes have been in use, a major breakthrough in anode advancement led to the development of the dimensionally stable anode, principally for use in the chlor-alkali industry. This anode relied typically on a coated valve metal. There then followed attempts to utilize concepts behind this advance so as to devise an improved lead electrode such as for copper electrowinning.
One conceptual approach was to unite in some way the desirable characteristics of a valve metal, e.g., the excellent acid resistance of a valve metal such as titanium, with the desirable features of the conventional lead anodes, including the presence of an oxide that can be an electroconductor. Using this approach, it was proposed to make a composite anode from a sintered article of one metal, e.g., the titanium, which article is infiltrated with the other metal, i.e., the lead. These anodes have been proposed, for example, in U.S. Pat. No. 4,260,470. The titanium can be ground, compressed and sintered to prepare a titanium sponge as a porous matrix. This matrix is then infiltrated with molten lead or lead alloy. The object is first to provide planar anodes in the form of strips. The strips are then joined together in a parallel, co-planar array to provide a large sheet anode.
The patent teaches employing these anodes particularly for use in electrowinning zinc or copper from sulfate electrolytes. However, if the sintered metal is infiltrated with lead, under the anodic conditions that are present such as in a copper electrowinning cell, the lead is anodically oxidized to lead dioxide. Thus, the anode can present loss of lead to the electrolyte, with resultant sludge build-up, and/or require electrolyte additives to deter such loss. Therefore, these anodes are ostensibly better suited for use in lead-acid batteries. Such utility has been disclosed in U.K. Patent Appln. No. 2,009,491A. In any event, there is today no known utilization of these anodes commercially such as in the copper electrowinning industry.
It has also been proposed to retain the commercially acceptable lead anodes, while fully utilizing the technical advance of the coated valve metal development. To this end, ways have been considered as to how to shield the lead from electrolyte, so as to reduce, to eliminate, lead erosion. Thus, it has been proposed to prepare catalytic particles of a metal such as titanium, which particles are activated with a platinum group metal. These particles are then uniformly distributed over, and partly embedded within, the surface of an anode base of lead or lead alloy. The lead plate is thus covered with a layer of these particles, such as of activated titanium sponge particles. Such an anode has been disclosed in U.S. Pat. No. 4,425,217. Therein it is taught that the anode offers improved electrochemical performance for anodically evolving oxygen in an acid electrolyte, and use is taught such as in the electrowinning of metals. However, it was found to be uneconomically viable to scale up this concept and to provide a uniform layer of small particles on the surface of commercial lead electrodes. In working with a multitude of particles, it was further found that the resulting article was difficult to refurbish. As a result, there is no known commercial use today of this anode.
Despite these developments, there then has not yet been found a commercially practicable anode, as a replacement for lead or lead alloy anodes, in industries such as copper electrowinning from sulfate electrolyte. Even today, decades after the development of the dimensionally stable anode for use in the chlor-alkali industry, the anode of choice for copper electrowinning is still the historical lead or lead alloy anode. There is thus a need for an anode, particularly for electrowinning of a metal, which is serviceable for extended stable operation. As an example of this need, even today it is not unusual to remove from 80 to 100 pounds of sludge, comprised principally of lead oxide and lead sulfate, after only about a week of operation, from a single commercial copper electrowinning cell that uses lead anodes. There is not only still the need for a commercially practicable as well as stable anode, but also the need for one which can be readily prepared for reuse and, in reuse, provide similar, extended operation. Therefore, it would be desirable to provide an anode, as either a fresh or refurbished anode structure, having stability, economy of operation, and economy of preparation as a fresh or refurbished structure.